1. Field
The following description relates to a burst-mode optical receiver and, more particularly, to a burst-mode optic pre-amplifier having an extended linear input range.
2. Description of the Related Art
Fiber to the home (FTTH) is a generic method of providing an optical line all the way to home in order to transmit more information more rapidly. A Passive Optical Network (PON) is developed to establish FTTH with low costs. In addition, various PON technologies, including EPON- and GPON-related ones, have been standardized or already used. The examples of the PON technologies include 10 Gbit/s Ethernet Passive Optical Network (10 G-EPON) 40-Gigabit-capable passive optical networks (NG-PON2).
A PON mainly consists of an Optical Line Terminal (OLT), an Optical Distribution Network (ODN), and an Optical Network Unit (ONU). As many subscribers use a single time-division multiplexed optical line, an ONU signal transmitted by each home composes predetermined time unit packet data and a combination of the unit packets including a null signal is temporally divided so as to be input to the OLT side. However, path loss occurs in a signal received by the OLT side due to a distance between the ONU side and the OLT side, so that packet signals have different size and phase. These signals are called burst data signals. To compensate for an inconsistent level of an optical input signal, a Burst Mode Trans-Impedance Amplifier (BM-TIA) is necessarily required in the front element of a receiving end on the OLT side. The BM-TIA has to have a wide dynamic range to compensate for signals of various size and prevent non-linear properties.
FIG. 1 is a block diagram illustrating a general BM-TIA using a feedback resistor.
Referring to FIG. 1, a general BM-TIA 1 is a combination of a Trans-Impedance Amplifier (TIA) 10 and a feedback resistor RF 12, and controls entire gains of an amplifier by adjusting a value of the feedback resistor RF 12. A peak detector 14 monitors an output voltage vout1 of a TIA 20 to compare the output voltage vout1 with a reference voltage Vref, and transmits information on whether to perform gain controlling to the feedback resistor RF. Gain controlling is generally performed in two steps, so an ON/OFF signal determining only a large or small gain is output. Since a gain is controlled by size of an input signal, a wider dynamic range may be secured, compared to the case where a single gain is controlled. If a soft burst packet signal of small size is input to the TIA 10, a value of the feedback resistor RF 12 is controlled to increase in an attempt to increase a gain. Alternatively, if a loud burst packet of large size is input, a value of the feedback resistor RF 12 is controlled to be reduced in an attempt to reduce a gain. The TIA 10, which is a current-voltage conversion amplifier, amplifies week current Iin that is a current signal which has been converted from an optical signal through a photo diode (PD). Then, the TIA 10 transmits, to a next circuit block, the amplified week current Iin in the form a signal of a voltage Vout1. Since a gain is determined by adjusting a value of the feedback resistor RF 12, circuit stabilization of the TIA 10 heavily depends on the value of the feedback resistor RF 12. Thus, a structure where a phase margin is dramatically changed by variation or a gain, thereby seriously affecting circuit stability, is not suitable for the TIA 10.
FIG. 2 is a diagram illustrating a circuit structure of a general common source-type BM-TIA.
Referring to FIG. 2, a common source type BM-TIA 2 is a more widely used circuit structure than a common gate type or common drain type BM-TIA. A common source type optical pre-amplifier using a feedback resistor RF 20 is capable of freely adjusting a gain and easily securing a wide dynamic range. However, the feedback resistor RF 20 on a current path may cause huge noise, and the common source type BM-TIA 2 does not have a wide bandwidth because of a large input parasitic capacitance. In addition, a BM-TIA is located at the front end in the OLT side, and noise figure and a bandwidth of the front end most affects the entire system. Thus, there is need for a new circuit structure which has lower noise figure and a wider bandwidth, compared to the existing common source structure.